The invention relates to a display device comprising a first substrate having a group of row or selection electrodes and a group of column or data electrodes and a matrix of picture electrodes arranged in rows and columns at a location of ferro-electric liquid crystal display elements between the first substrate and a second substrate provided with a counter electrode, each display element being connected to a column electrode via an active switching element and a display device comprising means for bringing, prior to selection, a row of display elements to an extreme optical transmission state by means of an auxiliary signal.
An extreme optical transmission state is herein understood to mean such a state that the pixel is substantially entirely or maximally transmissive or non-transmissive. This state is also determined by the type of ferro-electric liquid crystal material used (ferroelectric, anti-ferro-electric) and, for example the position of possible polarizers.
Such display devices, notably equipped with ferro-electric liquid crystal material are used, for example in television apparatus or in apparatus for non-volatile display. Advantages are the high switching rate of ferro-electric liquid crystal materials and their minor dependence on the viewing angle.
A display device of the type mentioned in the opening paragraph is described in U.S. Pat. No. 4,976,515. Prior to selection, the display elements, or pixels, are brought to an extreme state by means of the auxiliary signal To be able to use rapidly switching ferro-electric liquid crystal materials, the rows of pixels within a row selection period are first brought to the extreme state, (for example, the fully transmissive state) by means of the auxiliary signal (blanking) and subsequently they are selected while information to be displayed is presented simultaneously. When slower materials are used, the auxiliary signal or blanking signal may alternatively be supplied one or more row selection periods in advance, as described in U.S. Pat. No. 4,840,462.
Thin-film transistors (TFTs) are used as switches in said display devices. Via a column electrode, which is connected in an electrically conducting manner to the source zone of the transistor, and the drain zone of the transistor, the auxiliary signal is applied between a picture electrode connected in an electrically conducting manner to the drain zone and a common counter electrode. Ferro-electric liquid crystal material is present between the common counter electrode and the picture electrodes. The voltage at the picture electrode, hence at the column electrode, should have a sufficient amplitude to fully bring a pixel to its extreme transmission state.
While information is being written, the column electrode is subsequently provided with the suitable voltage. If the pixel must be brought to a state which is practically equal to the other (non-transmissive) extreme state, it must be possible for the voltage at the column electrodes to vary within a large range, for example from -7 V to +6 V, dependent on the liquid crystal materials used and the properties of the transistors. For most column drive circuits such a voltage swing cannot be realized or can only be realized at a very high cost.